DVB-T2 as described in the DVB-T2 standard “Digital Video Broadcasting (DVB); frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, ETSI EN 302 755 V1.1.1 discloses a plurality of so-called “modcods”. A modcod is a pair consisting of a modulation/signal constellation such as QPSK, 16-/64-/256-QAM and code rates (1/2, 3/5, 2/3, 3/4, 4/5, 5/6). Each modcod has associated a spectral efficiency. The spectral efficiency is, for example, for a modcod of 16-QAM and a code rate of 2/3 as follows: 4 codebits/symbol*2 infobits/3 codebits=8/3 infobits/symbol=2.67 bits/s/Hz. Additionally, a constellation rotation including a coordinate interleaving can occur subsequent to the mapping of codebits. This procedure is, for example, disclosed in “Jonathan Stott: ‘Rotated Constellations’ available from http://www.dtg.org.uk/dtg/t2docs/RotCon_Jonathon_Stott_BBC.pdf”. The advantage of such a constellation rotation is a higher diversity when transmitting the coded signal which has been mapped to a certain signal constellation. This results in a higher robustness for a given modcod and a spectral efficiency provided by the given modcod.
Typically, a DVB transmission comprises an FEC encoder for applying a certain forward error correction code to an information word. An information word may, for example, consist of 10,000 bits and advantageously consists of, for example, 1,000 bits to 100,000 bits. Depending on the code rate, the FEC encoder generates a codeword from the information word. When there is a code rate of, for example, 1/3, the codeword consists of 30,000 bits when the information word has 10,000 bits. For example, when the codeword has 100,000 bits, then the codeword will have 300,000 bits. The bits of the codeword are introduced into a subsequent bit interleaver. The bit interleaver performs an interleaving within the codeword only, i.e. the for example 300,000 bits within an encoded codeword are interleaved so that an interleaved codeword results, but bits from one codeword are not interleaved with bits of a different codeword. Then, subsequent to the bit interleaver, an interleaved codeword having codebits exists. The codebits are grouped depending on a certain constellation diagram applied in a constellation mapping procedure. When the constellation diagram is, for example, a 256-QAM constellation diagram, then groups of 8 codebits are formed in order to map this group of 8 codebits into a constellation symbol. In 64-QAM, only 6 bits are grouped and mapped to one of the 64 different QAM symbols. Depending on the implementation, a constellation rotation and a cyclic Q-delay can be applied to the individual symbols in order to obtain individual cells. However, the constellation rotation or the cyclic Q-delay can be dispensed with so that the symbols output by the constellation mapping are the same as the so-called cells in the context of the DVB standard. Then, cells are input into a cell and time interleaver to obtain interleaved cells. In the cell and time interleaver the number of cells making up a certain codeword are interleaved with cells from a different codeword, but no interleaving within the cells/modulation symbols themselves occurs. The individual modulation symbols are expressed as complex numbers, where each complex number has an in-phase component or I-component and quadrature component (Q-component). A pair of an I-component and a Q-component which are also called “data units” makes up a constellation symbol or cell. However, with constellation rotation and cyclic Q-delay, a cell is different from a symbol in that a Q-component of a different symbol is paired with a I-component of another symbol while, without the constellation rotation or a cyclic Q-delay, the paired I-component and Q-component of a cell actually make up the constellation symbol in the UQ plane. Then, subsequent to the cell and time interleaver, the interleaved cells are forwarded to a frame builder.
The FEC encoder performs a channel encoding. The bit interleaver is provided for destroying statistical dependencies which would be there in the receiver between the bits of a symbol, such as the 8 bits of a 256-QAM. These statistical dependencies would have a negative impact on the decoding of the channel codes. For example, when a 256-QAM-symbol would be heavily distorted, then 8 sequential bits would be non-decodable, and such a so-called burst error would result in a more negative impact when compared to a situation where the bit interleaving is applied.
The constellation is obtained, as discussed before, by a mapping of the codebits to a certain desired signal constellation such as 16-QAM.
The constellation rotation and cyclic Q-delay is optional. However, the following example clarifies the technology behind the cyclic Q-delay as described in the conventional-technology reference mentioned before.
[Before any Cyclic Delay]
Cell1-0I1Q1Cell2-1I2Q2Cell3-0I3Q3Cell4-0I4Q4[After Cyclic Delay of Length=4]
Cell1I2Q1Cell2I3Q2Cell3I4Q3Cell4I1Q4
The cell interleaver makes sure that the I and Q coordinates of a symbol are transmitted at different time instances and on different subcarriers of, for example, an OFDM signal (OFDM=orthogonal frequency division multiplex).
The time interleaver distributes the cells, which belong to an FEC codeword, over a certain time which is also called the interleaver time period. This provides time diversity. Time diversity is advantageous in that only a portion of an FEC codeword is strongly distorted when the transmission channel is not so good at a certain time instance. However, the remaining less distorted portion of the codeword might be sufficient for a successful decoding operation.
The frame builder builds the transmission frames, where a transmission frame defines the actual transmission signal for a predetermined time interval such as 200 ms. Since the T2 standard allows several physical layer pipes (PLPs), i.e. more parallel structures, but with individual modcods, the frame builder builds the frames from different output signals of several existing time interleavers. Such an individual processing chain is also called a “pipe” in the DVB context.
On the receiver side, the chain is processed in the reverse order. One of the blocks in the receiver is the time de-interleaver. The time de-interleaver operates in a cell-wise manner, wherein a cell can comprise, e.g., a received non-rotated QPSK or a rotated 256-QAM. A rotated 256-QAM has 256 possible values for the I-coordinate and additionally for the Q-coordinate. This means that a cell can have values such as a (transmitted in a noisy channel) 256*256-QAM=65 k-QAM. Since a cell can be any one of these constellations, it is useful to finely quantize the I- and Q-coordinates in the receiver before the I- and Q-coordinates are input into the time de-interleaver. In the DVB-T2-implementation guidelines: “Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T2)”, ETSI TR 102 831, it is outlined that one should apply a 10-bit quantization for the I- and Q-components and one should also provide several additional bits for the channel state information, i.e. for the information on an estimated signal-to-noise ratio (SNR) for this cell so that, in the end, one may use 24 to 30 bits per cell, where a cell comprises a pair of data units, i.e. an I portion as a first data unit, a Q portion as the second data unit and the channel state information bits.
It has been settled in DVB-T2 that a receiver may be in the position to de-interleave 219 cells. When one applies the above-referenced cell quantization such as 24 to 30 bits per cell, and when the number of cells within the receiver (219) is considered, the whole memory requirement for the time de-interleaver will be between 12 and 15 Mbits.
The number of cells within the time de-interleaver naturally determines the size of the time interleaver within the transmitter. The time interleaver within the transmitter also has to be in the position to process 219 cells.
The DVB-T2-standard is finalized and implemented in receivers and is actually used in several countries such as Great Britain and Sweden.
Actually, two further developments of DVB-T2 are under consideration. DVB-T2 mobile should be very close to T2, but should be better suited for a mobile reception within a car or a train. The DVB-Next Generation Handheld (NGH) has the same targets as T2 mobile, but is allowed to have larger deviations from the T2 standard.
One of the most important points in these developments is to try to reduce the price of the receiver chips as much as possible. Today's DVB-T2 receivers are quite expensive and therefore not suitable for being integrated into a mobile phone, since in the mobile phone industry the pressure on the manufacturer for a low price device is particularly high.
It has been found out that a very important contribution to the price of the receiver chip is the memory that may be used for the time de-interleaver, which is typically implemented as an on-chip-RAM.
Therefore, plans exists that T2 mobile and an NGH receiver should have a reduced memory size and should therefore be in the position to only de-interleave a correspondingly reduced number of cells. One example is to reduce the memory by a factor of two, which means only 218 cells can be de-interleaved. Correspondingly, the transmitter is also allowed to only interleave such a reduced number of cells.
A reduction of time interleaving, however, means that the cells, which belong to a codeword are spread over a shorter time period. In other words, the time diversity decreases. Therefore, the robustness or power efficiency also decreases in time-variant fading channels.